Not applicable.
The present invention relates to tools for electromagnetic induction well logging instruments. More specifically, the present invention relates to methods for obtaining resistivity measurements at greater depths and with better vertical resolution that has heretofore been possible. Still more particularly, the present invention relates to a logging tool and operating system therefor that provides high resolution resistivity measurements using a multi-receiver array and novel data processing techniques.
In petroleum drilling, it is often desirable to survey the formation using a logging tool lowered through the wellbore. Electromagnetic induction well logging instruments are used to make measurements of the electrical resistivity of earth formations penetrated by wellbores. Induction well logging instruments typically include a sonde having a transmitter coil and one or more receiver coils at axially spaced apart locations from the transmitter coil.
The basic element in all multi-coil induction tools is the two-coil sonde. The two-coil sonde consists of a single transmitter coil and a single receiver coil wrapped around an insulating mandrel. The transmitter coil is driven by an oscillating current at a frequency of a few tens of kilohertz. The resulting magnetic field induces eddy currents in the formation which are coaxial with the tool. These eddy currents produce a magnetic field which in turn induces a voltage in the receiver coil. This induced voltage is then amplified, and the component of the voltage that is in-phase with the transmitter current is measured and multiplied by a tool constant to yield an apparent conductivity signal. This apparent conductivity is then recorded at the surface as a function of the depth of the tool.
The two-coil sonde has several practical limitations. Its response is adversely affected by several factors including the borehole, adjacent beds, and mud filtrate invasion. Also, the two-coil sonde is difficult to implement because of the large direct mutual coupling between the coils. Even though this mutual signal is out of-phase with the transmitter current, it is a problem because a very small phase shift in the electronics can cause this mutual coupled signal to xe2x80x9cleakxe2x80x9d into the apparent conductivity signal. For these reasons, it is the standard practice in the industry to construct induction logging tools with coil arrays which include additional coils. Typically, there are several transmitter coils and several receiver coils. In certain applications, all of the transmitter coils may be connected in series into one circuit. Similarly, all of the receiver coils may be connected in series in a separate circuit. The additional coils served to cancel out the various adverse effects listed above. Such arrays are generally termed xe2x80x9cfocused arrays.xe2x80x9d
The following are terms of art that are used often to compare various induction tools. The xe2x80x9cvertical resolutionxe2x80x9d of a tool is a measure of the thinnest bed that a tool can detect. That is, a tool may accurately estimate the thickness of beds that are thicker than its vertical resolution. A tool can also accurately locate a bed boundary to within the tolerance of its vertical resolution. There is still a significant error in the apparent conductivity reading in a thin bed, which is attributable to signals from adjacent beds; however, so long as the thin bed is thicker than the vertical resolution of the tool, the tool can estimate the thickness of the bed. The error in the apparent conductivity reading of a thin bed attributable to signal from adjacent beds is referred to as xe2x80x9cshoulder effect.xe2x80x9d In known induction tool arrays, the additional coils are arranged to cancel out much of this shoulder effect.
It is also possible for a tool to have good vertical resolution but poor shoulder effect. Such a tool would be able to accurately define bed boundaries but would give poor estimates of the conductivities of these thin beds. Vertical resolution and shoulder effect are two aspects of the vertical focusing of an induction tool coil array.
The xe2x80x9cdepth of investigationxe2x80x9d of a tool is a measure of the average radius of penetration of the signal. The xe2x80x9cdepth of investigationxe2x80x9d is defined as the radius of the cylinder from which half the apparent conductivity signal comes. The xe2x80x9cborehole effectxe2x80x9d is a measure of how much signal comes from the borehole as compared to the formation. In conventional arrays, coils are arranged to cancel much of the signal coming from near the tool so that the xe2x80x9cdepth of investigationxe2x80x9d will be large and the xe2x80x9cborehole effectxe2x80x9d will be small.
The foregoing discussion is based on an assumption that a tool can be operated at a sufficiently low frequency that there is no significant attenuation of the transmitted signal as the signal propagates through the formation. In practice, such attenuation cannot be neglected, since it reduces the transmitted signal proportionately more in conductive formations. The voltage actually induced in the receiver coils is typically less than what would be induced for any value of conductivity were the relationship between eddy current magnitude and the induced voltage a linear one. The difference between the voltage actually induced and the voltage that would have been induced if the relationship were linear results from the so-called xe2x80x9cskin effect.xe2x80x9d Prior art practitioners generally attempt to design a coil array which has moderate skin effect at the highest conductivity of interest in logging situations and then correct for the skin effect at the surface. The skin effect correction is typically a correction which yields the true conductivity of a homogeneous formation.
In the case of conventional induction tool arrays, coils must be positioned to define the tool""s vertical resolution, depth of investigation, as well as to compensate for borehole and shoulder effect. In addition, the coils must minimize the mutual coupling between transmitter coils and receiver coils, as this signal is very large when compared to most formation signals. In known coil arrays, the position and strength of each coil controls each of these aforementioned effects. Because each of these effects may change as a coil is modified, it is difficult to design a coil array optimized to reduce all of these effects simultaneously. The different effects interact, as one effect is reduced, another is increased. Conventional coil array designs therefore must be a compromise between sharp vertical resolution and deep radial penetration into the formation. In addition, in some prior art tools, the deep measurements lack sufficient vertical resolution, so the high resolution shallow measurements are used to enhance the resolution of the deep coil measurements. This is undesirable, however, particularly when the shallow measurements become corrupted. It would be desirable, therefore, to provide an induction logging tool that permits both sharp vertical resolution and deep radial penetration.
In most commercial applications, it is also desired to investigate the strata surrounding a borehole to different depths, in order to determine the diameter of invasion of the strata by borehole fluids. This requires at least two measurements with contrasting radial response and ideally identical vertical resolution so that differences in the logs obtained will be due to radial anomalies in the formations, such as invasion. Most prior art dual induction tools use deconvolution filters to match dissimilar vertical responses of two induction coil arrays with inherently different vertical resolutions by smoothing out the response of the array with the sharper vertical resolution and degrading it to match the vertical response of the second array. This approach is not desirable in view of the degradation of vertical resolution that is needed to match the different coil arrays.
In another prior art system, separate deep transmitter coils and medium-deep transmitter coils are provided. Because the cross-talk would obscure any signal received from either set of transmitter coils, it is necessary to use a time multiplexing approach. In the time-multiplexing approach, the sets of transmitters are turned on alternately and a settling period is allowed between signals, so that only one signal path is in use at any time. The time-multiplexing approach becomes less practical, however, as the number of coils increases, and is impractical when six or more sets of transmitter coils are used.
Other prior-art systems for array induction logging have a single transmitter coil and sets of receiver coils above and/or below the transmitter. Each set of receiver coils consists of a main receiver coil and a bucking receiver coils. The main coils are positioned at different distances from the transmitter and the coil sets are arranged such that the distances between the main and bucking receivers increase with increasing distance from the transmitter. In such a system, the vertical resolution for the deeper arrays is inferior to the vertical resolution of the shallower arrays. With such a system, the measurements from the deep arrays must be combined with information from the shallow arrays in order to produce a deep response with good vertical resolution. The shallow arrays are more affected by temperature and the borehole, and this system causes errors in the data generated by the shallow arrays to degrade the accuracy of the deep logs.
Thus, a need exists for an array induction tool capable of investigating multiple depths of investigation while maintaining substantially identically vertical resolution for all coil arrays. It is further desirable to provide a tool capable of separating the vertical and radial aspects of the signal processing. A more detailed discussion of these and related problems can be found in U.S. Pat. No. 5,065,099, which is incorporated herein in its entirety.
The present invention includes a coil array and signal processing system that permit sharp vertical resolution and deep radial penetration. Further, the present system is capable of investigating multiple depths of investigation while maintaining substantially identically vertical resolution for all coil arrays. The present coil array and signal processing system allow 10, 20, 30, 60, 90 and 120 inch depths of investigation, with of one and two foot vertical resolutions.
The present tool includes a plurality of coils spaced along the tool body at preferred intervals. Several of the coils are shared between coil sets and several of the coils may be tapped, so that, for example, ten elemental measurements can be made by 19 coils. The size, spacing and direction of winding of the coils allows the present signal processing system to calculate a weighting system that yields conductivity measurements for the preferred depths of investigation. The preferred signal processing system digitized the received waveforms and extracts phase information from the digitized signal.
According to a preferred embodiment, each elemental measurement is deconvolved vertically to match resolution before any radial combination occurs. Further according to the present invention, the output deep measurements (90 and 120 inch depths of investigation) are constructed from only the deepest of the elemental measurements and are not corrupted with the shallow elemental measurements.